1. Field of the Invention
The present invention relates to methods for manufacturing battery cell electrodes. In particular, the invention relates to an electrochemical method for roughening the surface of an electrode substrate to provide a greater amount of contact area for active material.
2. Description of Related Art
A continuing problem with batteries is their large size and weight. Many systems demand batteries with a smaller mass and increased performance. In particular, the mass, size and performance of batteries is critical for applications in space. The mass and size are significant because of the tremendous launch and payload costs for spacecraft. Performance is important because batteries in spacecraft are commonly either very difficult or impossible to replace. The problem is heightened by the increased power requirements of many satellites an space stations. The trend of past power requirements for spacecraft has been steady growth, and this trend is expected to continue for the larger and more powerful spacecraft now being developed. Thus, there is a need for a more effective energy storage system that will satisfy the projected power requirements of future spacecraft.
The electrodes are particular components of battery cells that limit the energy density of batteries. The voltage performance of a battery cell electrode is limited by the amount of contact area between the active material and the electrode substrate or plaque, because voltage performance is affected by the current density across this interface. In particular, IR drop (resistance-induced voltage loss) and activation polarization (energy barrier controlled voltage loss) during charge/discharge cycling of the battery are difficult to reduce with present methods for constructing battery cell electrodes that limit the amount of active material contact area. Also, concentration polarization (diffusion limited voltage loss) of the positive electrode has diminished the performance of battery cell electrodes because of poor active material contact area.
Porous plaques have been used to improve the performance of batteries. The use of porous plaques greatly increases the contact area for the deposition of active material. However, the microsurface of the plaque tends to be very smooth, and therefore, surface area is not optimized. The smooth microsurface of plaques also makes it difficult to hold the active material in and on the plaque. Cell life is drastically reduced because the active material within the electrodes is often extruded from the pores of the plaque after continued operational cycling of the battery.
Various methods for making electrodes using porous plaques or substrates have been developed as disclosed in U.S. Pat. Nos. 3,335,033, 3,507,699, 3,523,828, 4,132,606, 4,292,143, 4,554,056 and 4,863,484. Porous "plaques" or "substrates" are electrically conductive structures that support an active material such as nickel hydroxide. Typically, they are flat thin pieces of porous nickel constructed by sintering pure nickel powder on a nickel screen. After the plaque has been formed, it often is electrochemically impregnated with active material by placing the plaque in a bath with an applied electrical potential. The methods disclosed in the above patents are directed to improving the performance of batteries with different processes for impregnating the plaque with active material. These methods use various baths and different voltage levels for impregnation. Nonetheless, a need for higher energy density than provided by electrodes constructed in accordance with the methods disclosed above continues to exist.
Some methods for increasing electrode voltage performance attempt to increase the amount of surface area on electrode substrates for active material contact by controlling the nickel powder morphology and sintering process. However, these methods have been unable to provide substantially more area on the plaque for impregnation without significantly weakening the mechanical strength of the plaque. A method is needed for increasing plaque surface area without significantly reducing plaque strength.
The present invention utilizes an electrochemical method for increasing the surface area (roughening) of the plaque. Other electrochemical processes causing surface roughening to a limited extent have been disclosed. In particular, Arvia, et al., disclose facetting platinum through an electrochemical process in The Electrochemical Facetting of Metal Surfaces: Preferred Crystallographic Orientation and Roughening Effects in Electrocatalysis. 20 Journal of Applied Electrochemistry 347-356 (May 1990); and Review Article Electrochemical Facetting of Metal Electrodes. 31 Electrochimica Acta 1359-1368 (Nov. 1986). It should be noted that electrochemical facetting is very different from surface roughening. Facetting or preferred crystallographic orientation of metal particles may be accomplished without any resulting surface roughening. Other articles related to electrochemical facetting also include: Perdriel, et al., Different Processes Contributing to the Development of Preferred Oriented Platinum Surfaces by Fast Periodic Potential Perturbation Techniques, 205 Journal of Electroanalytical and Interfacial Electrochemistry 279-290 (Jun. 25, 1986); Triaca, et al., A Study of the Optimal Conditions for the Development of Preferred Oriented Platinum Surfaces by Means of Fast Square Wave Potential Perturbations, 134 Journal of Electrochemical Society 1165-1172 (May 1987); Albano, et al., A Mechanistic Model For the Electrochemical Facetting of Metals with Development of Preferred Crystallographic Orientations, 33 Electrochimica Acta 271-277 (Feb. 1988); and Cervino, et al., A Novel Effect. Changes in the Electrochemical Response of Polycrystalline Platinum Promoted by Very Fast Potential Perturbations, 132 Journal of Electrochemical Society 266-67 (1985). In these publications, there is some indication that surface roughening which increases the contact area for active material, may result from the application of fast periodic potential perturbations (i.e., greater than 1000 Hz). However, roughening does not consistently result, and there is no suggestion or disclosure that such methods are applicable to the construction of battery cell electrodes. Moreover, there is no suggestion that facetting or roughening may be performed on porous plaques or substrates. In particular, Arvia's work is limited to planar electrodes and the applicability of facetting techniques to porous sintered nickel plaques is not considered in any of the papers. Also, nickel, a primary metal in the construction of battery electrodes, is omitted from the processes disclosed by Arvia, et al. in the above articles with the exception of the article entitled "The Electrochemical Facetting of Metal Surfaces: Preferred Crystallographic Orientation and Roughening Effects in Electrocatalysis." However, this paper was published in May 1990, after the date of conception and reduction to practice of the present invention by the Applicants. The present invention is also distinguishable from the above papers which recommend fast potential perturbations because the present invention uses perturbations below 1000 Hz. Also, the techniques disclosed in the articles do not reveal fluctuating the voltage between the voltages for electrodissolution and electrodeposition of the plaque, but rely on electroadsorption and electrodesorption. The technique disclosed by Arvia is further distinguishable from the present invention because Arvia's work is absent any suggestion that the temperature of the solution can affect roughening. In fact, there is no suggestion or disclosure of elevating the temperature of the solution.